Stockholders' Equity - Common stock, resale registration statement, and follow-on public offering (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Class Of Stock [Line Items]
Common Stock Shares Authorized	100,000,000		100,000,000		100,000,000
Common stock, par value	$ 0.0025		$ 0.0025		$ 0.0025
Other offering expenses			$ 117
Reclassified out of accumulated other comprehensive loss for realized gains or losses on available for sale securities	$ 0	$ 0	0	$ 0
Accumulated deficit subject to restriction			$ 8,700		$ 8,700
Minimum
Class Of Stock [Line Items]
Accumulated profits			10.00%		10.00%
Employee Stock Options
Class Of Stock [Line Items]
Common stock	7,807,121		7,807,121
Sale of common stock upon the exercise of overallotment option			402,977
Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock	426,217		426,217